PbI2 Crystals Research Articles

Regulation of Lead Iodide Crystallization and Distribution for Efficient Perovskite Solar Cells.

At present, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has reached 26.1%. Polycrystalline perovskite films prepared by sequential deposition are often accompanied by excess PbI2. Although excess PbI2 can reduce the internal defects of the perovskites and promote charge transfer, excess PbI2 is unevenly distributed in the perovskites and easily decomposed into the composite center of charge. Therefore, the growth and distribution of PbI2 crystals can be regulated by introducing 4-fluoroaniline (4-FLA) as an additive into the precursor of PbI2. We observe that the presence of an amino group in 4-FLA leads to a reduction in the strength of van der Waals forces between PbI2 layer structures, thereby facilitating the uniform dispersion of excess PbI2 within the perovskites. Additionally, 4-FLA is restricted from being embedded in the PbI2 layer due to the steric hindrance of 4-FLA and the hydrogen bond interaction between nitrogen atoms and PbI2. Therefore, it leads to better dispersion of PbI2, resulting in better passivation and device efficiency. Based on the hydrophobicity of the benzene ring, the modified perovskite film shows excellent hydrophobicity. Ultimately, we achieved 21.63% PCE and 1.16V VOC. This provides an effective strategy for regulating excess PbI2 to achieve efficient and stable PSCs.

Perovskite Crystallization and Hot Carrier Dynamics Manipulation Enables Efficient and Stable Perovskite Solar Cells with 25.32% Efficiency

AbstractModulating perovskite crystallization and understanding hot carriers (HCs) dynamics in perovskite films are very critical to achieving high‐performance perovskite solar cells (PSCs). Herein, a small organic molecule (6BAS) with multisite anchors (C═O) as an efficient additive is introduced into PbI2 precursors to modulate perovskite crystallization during two‐step sequential deposition. The chemical interaction between 6BAS and PbI2 enables more preferential PbI2 crystal with enlarged interplanar spacing of PbI2 lattice, which is beneficial to the penetration of organic ammonium salts into PbI2 layer and the complete conversion to perovskite, consequently promoting the preferential crystallization of perovskite to realize high‐quality perovskite films with larger grain size and reduced defect state. By ultrafast spectroscopy, it is found that the incorporation of 6BAS can efficiently prolong HCs cooling, which helps to enhance HCs transfer and retard the charge carrier recombination in device. As a result, 6BAS doped‐PSCs efficiency significantly enhances to 25.32% from 22.91%. The target device achieves the enhanced long‐term stability. Only a 6% efficiency degradation is realized for un‐encapsulated device with 6BAS after 70 days under N2. Meanwhile, the 6BAS‐treated device retains 95% of its initial PCE after 1160 h of operation at the maximum power point under continuous AM 1.5 G illumination.

Growth of millimeter-sized 2D metal iodide crystals induced by ion-specific preference at water-air interfaces

Conventional liquid-phase methods lack precise control in synthesizing and processing materials with macroscopic sizes and atomic thicknesses. Water interfaces are ubiquitous and unique in catalyzing many chemical reactions. However, investigations on two-dimensional (2D) materials related to water interfaces remain limited. Here we report the growth of millimeter-sized 2D PbI2 single crystals at the water-air interface. The growth mechanism is based on an inherent ion-specific preference, i.e. iodine and lead ions tend to remain at the water-air interface and in bulk water, respectively. The spontaneous accumulation and in-plane arrangement within the 2D crystal of iodide ions at the water-air interface leads to the unique crystallization of PbI2 as well as other metal iodides. In particular, PbI2 crystals can be customized to specific thicknesses and further transformed into millimeter-sized mono- to few-layer perovskites. Additionally, we have developed water-based techniques, including water-soaking, spin-coating, water-etching, and water-flow-assisted transfer to recycle, thin, pattern, and position PbI2, and subsequently, perovskites. Our water-interface mediated synthesis and processing methods represents a significant advancement in achieving simple, cost-effective, and energy-efficient production of functional materials and their integrated devices.

Enhancing Organic–Inorganic Perovskite Thin Film Crystallization via Vapor–Solid Reaction by Modifying PbI2 Precursors Films with Pyridinium Trifluoromethanesulfonate

The vapor–solid reaction technique holds potential for producing large‐scale organic–inorganic perovskite films, using a two‐step deposition process. The sucess of perovskite formation heavily relies on the crystallization of lead iodide (PbI2) precursor films. However, the high crystallinity of PbI2 can hinder organic vapors' diffusion and negatively affecting the solar cells by increasing charge recombination and accelerating degradation. To address this, pyridinium trifluoromethanesulfonate (PTFS) is introduced into the PbI2 precursor solution to control the nucleation and growth of PbI2 films. PTFS has been found to effectively suppresses PbI2 crystallization and enhance the integration of formamidinium ion (FA+) through hydrogen bonding, facilitating the full transformation of PbI2 into perovskite. It also minimizes defect formation by interacting with uncoordinated Pb2+ and FA+ ions. These modifications have significantly enhanced power conversion efficiency (PCE), reaching up to 21.19%, and the devices have maintained 93% of their initial performance after 1580 h of exposure to air.

Columnar Liquid Crystal Enables In‐Situ Dispersing of Excess PbI2 Crystals for Efficient and Stable Perovskite Solar Cells

AbstractExcess PbI2 has been deemed as indispensable component to boost the efficiency of perovskite solar cells (PSCs). However, the random aggregation of PbI2 crystals seriously disturbs the transport behavior of the carrier and accelerates the degradation of perovskite film. Herein, an effective strategy to in situ disperse excess PbI2 crystals via the columnar liquid crystal (T6TE) is developed. Rapid self‐assembly induced by intermolecular π–π interaction enables T6TE to form the ordered columnar phase with “edge‐on” orientation in perovskite. The columnar T6TE can efficiently disperse excess PbI2 crystals with the merits of strong negative electrostatic potential and highly steric hindrance. Target perovskite deserves preferable crystallization and reconstructed surface, leading to reduced defects density, less residual stress, and efficient carrier transport. Besides, the T6TE significantly impedes the degradation of perovskite film and the formation of Pb0 defects. Resultant T6TE‐assisted PSCs achieve the champion power conversion efficiency (PCE) of 24.27% for mixed Cs+‐FA+‐MA+ perovskite. The PCE of a larger area (1 cm2) device reaches to 21.50%. The unencapsulated device maintains ≈85% of the initial PCE after 1500 h storage in the atmosphere with 40–60% relative humidity. This work provides a new strategy to in situ disperse excess PbI2 by incorporating columnar liquid crystal for the first time.

Growth by the facile vertical Bridgman method and optoelectronic properties of one-inch PbI2 crystal

Lead iodide (PbI2) single crystal is a promising semiconductor radiation detection material at room temperature. In this paper, one-inch PbI2 single crystal with the size of Φ25 mm × 60 mm has been successfully grown by the facile vertical Bridgman method, and SEM, XRD, UV–vis, Raman analyses were performed on the (001) plane. The Au-PbI2-Au Schottky structured PbI2 single crystal X-ray photodetector was prepared by magnetron sputtering method. The photodetector has a fast, repeatable and stable response to X-ray incident light with reasonable response time of the short rising and decay times, which were 0.112 and 0.331 s, respectively. We further illustrate the operation of the photodetector under different bias voltages and X-ray irradiation, confirming that PbI2 crystal is a promising photodetection material.

Novel Bilayer SnO2 Electron Transport Layers with Atomic Layer Deposition for High-Performance α-FAPbI3 Perovskite Solar Cells.

Perovskite solar cells (PSCs) based on the SnO2 electron transport layer (ETL) have achieved remarkable photovoltaic efficiency. However, the commercial SnO2 ETLs show various shortcomings. The SnO2 precursor is prone to agglomeration, resulting in poor morphology with numerous interface defects. Additionally, the open circuit voltage (Voc ) would be constrained by the energy level mismatch between the SnO2 and the perovskite. And, few studies designed SnO2 -based ETLs to promote crystal growth of PbI2 , a crucial prerequisite for obtaining high-quality perovskite films via the two-step method. Herein, we proposed a novel bilayer SnO2 structure that combined the atomic layer deposition (ALD) and sol-gel solution to well address the aforementioned issues. Due to the unique conformal effect of ALD-SnO2 , it can effectively modulate the roughness of FTO substrate, enhance the quality of ETL, and induce the growth of PbI2 crystal phase to develop the crystallinity of perovskite layer. Furthermore, a created built-in field of the bilayer SnO2 can help to overcome the electron accumulation at the ETL/perovskite interface, leading to a higher Voc and fill factor. Consequently, the efficiency of PSCs with ionic liquid solvent increases from 22.09% to 23.86%, maintaining 85% initial efficiency in a 20% humidity N2 environment for 1300 h.

Orientation-controlled mesoporous PbI2 scaffold for 22.7% perovskite solar cells

Sequential deposition is a promising tactic for perovskite film fabrication. However, the highly crystalline and compact morphology of PbI2 film will hinder the quality and reproducibility of perovskite films. Here, nicotinamide (NTM) derivatives were introduced as simple and effective multifunctional ligands into the PbI2 precursor solution. Orientation-controlled and mesoporous PbI2 scaffolds formed due to strong chemical interaction and accelerated solvent volatilization. Nanobelt PbI2 crystals were also generated at the grain boundaries of perovskite films with NTM derivatives, which can passivate the defects and enhance the carrier lifetime. As a result, the perovskite solar cells based on 6-(trifluoromethyl) nicotinamide-modulated films showed an enhanced power conversion efficiency of 22.7%, of which 98.8% was maintained even after 2000 h of aging. Our findings offer a distinctive understanding of regulating the crystallization of PbI2 films and manipulating excess PbI2 structure for efficient perovskite solar cells.

Tailoring particle size of PbI2 towards efficient perovskite solar cells under ambient air conditions.

PbI2 films as templates are essential to the quality of perovskite (PVK) film in a two-step sequential deposition process. Herein, we demonstrate that PbI2 microcrystal powder (P-PbI2) of micrometer size is beneficial for preparing higher-quality PbI2 films in contrast to millimeter-sized PbI2 crystals (C-PbI2) under low relative humidity (RH) conditions. Surprisingly, C-PbI2 allowed the growth of denser films than P-PbI2 under heavy RH conditions. Ultimately, P-PbI2 gave PVK solar cells (PSCs) a best power conversion efficiency (PCE) of 23.30% under a low RH of 30 ± 5%, and C-PbI2 derived an impressive PCE of 21.09% when fabricated under conditions with RH = 50 ± 5%. This work provides ideas for the selection of lead iodide for the fabrication of PSCs under air conditions.

Passivating Defects at the Bottom Interface of Perovskite by Ethylammonium to Improve the Performance of Perovskite Solar Cells.

The interface of perovskite solar cells (PSCs) plays a significant role in influencing their performance, yet there is still scarce research focusing on their difficult-to-expose bottom interfaces. Herein, ethylammonium bromide (EABr) is introduced into the bottom interface and its passivation effects are studied directly. First, EABr can improve substrate wettability, which is beneficial for the perovskite-film deposition. By lifting off the perovskite film spontaneously from the substrate, it is found that EABr can significantly reduce the amount of unreacted PbI2 at the bottom interface. These PbI2 crystals have been recently identified as a major defect source and degradation site for perovskite film. Meanwhile, EABr also lifts the valence band maximum at the bottom side of perovskite from -5.38 to -5.09eV, facilitating better hole transfer. Such a improvement is also verified by the study of charge carrier dynamics. Through introducing EABr, all photovoltaic parameters of the inverted PSCs are improved, and their power conversion efficiency (PCE) increases from 20.41% to 21.06%. The study highlights the importance of direct characterization of the bottom interface for a better passivation effect.

A Novel Method to Control the Crystallographic‐Preferred Orientation of Lead Iodide Toward Highly Efficient and Large‐Area Perovskite Solar Cells

Perovskite Films In article number 2200609, Pei-Ting Chiu and co-workers found that the proportion of the facet (001) of PbI2 crystals increases as the synthesis temperature increases and fabricates better formamidinium-based perovskite films. The perovskite solar cells based on the PbI2 synthesized at 120 °C exhibit the best power conversion efficiency (PCE) of 17.96% and the module with an active area of 3.68 cm2 shows a PCE of 16.08%.

Mandelic Acid as an Interfacial Modifier for High Performance NiOx‐based Inverted Perovskite Solar Cells

AbstractNickel oxide (NiOx) inverted perovskite solar cells have been witnessed as promising solar cells because of the low cast, easy fabrication and negligible hysteresis. However, the inferior interface feature of the link NiOx and perovskite layer often causes poor device property and repeatability. Here, Mandelic acid (MAD) is introduced to form a modified layer, which could improve the poor interface properties between perovskite layer and NiOx layer. The contact angel images revels MAD could increase the hydrophobicity and facilitate the growth of perovskite crystal. The X‐ray diffraction pattern and Fourier transform infrared spectroscopy have confirmed that the MAD small molecule could interact with residual PbI2 crystal, leading to better interface contact and high crystallinity. This is further demonstrated by the scanning electron microscopy test of NiOx/MAD/MAPbI3 film that average grain size is increased from 201.85 nm to 315.17 nm. The space‐charge limited current measurement proves the introduction of MAD molecule that result in the decreased trap state density from 3.58×1015 cm−3 to 2.721015 cm−3. As a consequence, The power conversion efficiency is significantly improved from 14.9% to 18.42% of MAPbI3‐based devices that retained nearly 80% initial PCE after 50 days in N2 without encapsulation.

Chemical Polishing of Perovskite Surface Enhances Photovoltaic Performances.

The benefits of excess PbI2 on perovskite crystal nucleation and growth are countered by the photoinstability of interfacial PbI2 in perovskite solar cells (PSCs). Here we report a simple chemical polishing strategy to rip PbI2 crystals off the perovskite surface to decouple these two opposing effects. The chemical polishing results in a favorable perovskite surface exhibiting enhanced luminescence, prolonged carrier lifetimes, suppressed ion migration, and better energy level alignment. These desired benefits translate into increased photovoltages and fill factors, leading to high-performance mesostructured formamidinium lead iodide-based PSCs with a champion efficiency of 24.50%. As the interfacial ion migration paths and photodegradation triggers, dominated by PbI2 crystals, were eliminated, the hysteresis of the PSCs was suppressed and the device stability under illumination or humidity stress was significantly improved. Moreover, this new surface polishing strategy can be universally applicable to other typical perovskite compositions.

Microspacing In-Air Sublimation Growth of Thickness-Controllable Lead Halide Crystal and the Morphology Evolution in Conversion to Perovskite

The formation process of hybrid perovskites affects the morphology, performance, and stability of the materials to a large extent. This work takes the preparation of a prototypical perovskite, methylammonium lead iodide (MAPbI3) from vapor conversion to illustrate the effect of reaction conditions. First, we applied our recently invented microspacing in-air sublimation (MAS) to grow PbI2 crystals with thicknesses ranging from several to hundreds of nanometers. The PbI2 single crystals were employed to investigate the conversion to MAPbI3. This proved that vapor conversion is indeed a single-crystalline-to-polycrystalline process, following a deconstruction–reconstruction mechanism. Both the crystal thickness and activation temperature greatly affect the revolution, while the morphology of the formed perovskite is dictated by the thickness of the precursor crystal. Clarification of the intrinsic and exerted factors on the conversion process is conducive to a deep understanding of the perovskite formation process.

PbI2-TiO2 Bulk Heterojunctions with Long-Range Ordering for X-ray Detectors.

High-performance X-ray detectors are usually based on single crystals, due to the long-range ordering and hence outstanding electronic properties. On the other hand, bulk heterojunctions (BHJs) that can effectively enhance photogenerated exciton dissociation are widely used for photodetectors. The benefits of both spur investigation into how to combine these two strategies to enhance X-ray detection. Here, TiO2 networks are incorporated into PbI2 crystals to form interpenetrating type II heterojunctions, namely BHJs. These BHJs exhibit long-range ordering in molecular packing similar to that of single crystals. Compared with single crystals, the long-range ordered BHJs facilitate the separation of photogenerated electrons and holes to inhibit recombination, extend the mobility lifetime product by 6.4 times, and consequently improve X-ray sensitivity by 5.8 times. Hence, this work provides a new strategy using gel-grown crystals to fabricate high-performance X-ray detectors as well as a new platform for studying the behavior of X-ray-generated carriers in BHJs with long-range ordering.

In Situ Management of Ions Migration to Control Hysteresis Effect for Planar Heterojunction Perovskite Solar Cells

AbstractAs one of the most promising photovoltaic materials, the efficiency of inorganic–organic hybrid halide perovskite solar cells (PSCs) has reached 25.5% in 2020. However, the stability and hysteresis remain primary challenges before it can become a commercial photovoltaic technology. Therefore, those issues have drawn significant attention for photovoltaic applications. In this work, a study of the PSCs hysteresis improvement is presented based on a combination of first‐principles simulations, scanning electron microscopy images, and time‐dependent photocurrent measurements. It indicates the hysteresis led by the ion migration and accumulation is mainly localized at the two interfaces: one is between electron transport layer and active layer, and the other is between active layer and hole transport layer. Considering the massive defects at the grain boundaries (GBs), they lower the potential barriers significantly. The defect density at GBs is therefore reduced via the in situ passivation of PbI2 crystals. The hysteresis index is decreased from 22.43% down to 1.04%, and results in an improvement in efficiency from 17.12% up to 20.10%. Following the understanding of defect‐induced hysteresis, an approach to improve the hysteresis is provided, which can be integrated into the fabrication process and widely applied to enhance the performance of PSCs.

Mesoporous TiO2 electron transport layer engineering for efficient inorganic-organic hybrid perovskite solar cells using hydrochloric acid treatment

We demonstrated effects of wet chemical processes using hydrochloric on crystallite structures, optical and electronic properties, and surface morphology of TiO2 thin films and enhancement in photovoltaic performance of perovskite solar cell using HCl-treated mesoporous TiO2 electron transport layers (ETLs). By treating mesoporous TiO2 thin films with HCl, hydroxyl groups on the surface were chemically neutralized and the film porosity was enhanced. Furthermore, the crystallite structure analyses indicated that CH3NH3PbI3 perovskite thin films deposited on the HCl-treated compact and mesoporous TiO2 ETLs possessed the relatively high crystallization along with avoiding formation of unexpected PbI2 crystals. As a result, the best power conversion efficiencies of devices using the HCl-treated mesoporous TiO2 ETLs were improved to 18.4% under forward bias scans.

Bendable 3D-structured x-ray photodetectors based on pure PbI2 single crystal

PbI2 crystal is a typical layered material with a I–Pb–I monolayer as a repeating unit. Benefiting from the coexistence of ionic and van der Waals bonding, the crystal exhibits low hardness and a low Young’s modulus, slips easily along the a–b plane, and is endowed with inherently high flexibility. In this work, bendable three-dimensional (3D)-structured x-ray photodetectors based on PbI2 crystal were prepared, and finite element simulation was employed to reveal the von Mises stress distribution from the surface to the bulk of the crystal, indicating the bending stress associated with the crystal’s thickness. The uniformly distributed electric bias field inside the crystal constrains the carrier transport process, provides a consistent geometric variation and electrical contact between the crystal and the electrodes, and further ensures a high effective collection area and enhanced collection capacity for photocarriers, even when the device is subjected to bending. The degradation percentage of the sensitivity after 100 bending cycles is 12.7% and 6.6% for thick (100 μm) and thin (10 μm) samples, respectively. All the simulated and experimental results indicate that PbI2 crystal, when used to construct a 3D-structured device of appropriate thickness, is a promising candidate for the development of flexible x-ray photodetectors.

Enhancing the Photovoltaic Performance and Moisture Stability of Perovskite Solar Cells Via Polyfluoroalkylated Imidazolium Additives.

Although the power conversion efficiency of perovskite solar cells has reached 25.5%, their long-term stability is still a barrier to commercialization. In this work, 1-methyl-3-(3',3',4',4',4'-pentafluorobutyl)imidazolium tetrafluoroborate (MFIM-2) ionic liquid and another two analogues were used as additives to study their interaction mechanism with the FAPbI3 perovskite layer. The results reveal that MFIM-2 suppressed the formation of PbI2 crystals during crystallization, enlarged the grain size, and reduced the defect density, which led to an increased photovoltage of 1.12 V and efficiency of 19.4%. Furthermore, the moisture stability of the solar cell devices was also improved. Devices with MFIM-2 retained above 83% of the original value after 35 days in an atmosphere with about 25% relative humidity, and the perovskite film with MFIM-2 showed no phase transition in a 10 month aging process. These results demonstrate that the additive strategy of the polyfluoroalkylated imidazolium salt is a promising way for simultaneously extending the lifetime and improving the device performance of the perovskite solar cells.

Detrimental Effect of Unreacted PbI2 on the Long-Term Stability of Perovskite Solar Cells.

Excess/unreacted lead iodide (PbI2 ) has been commonly used in perovskite films for the state-of-the-art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI2 has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI2 crystals under continuous light illumination. Unreacted PbI2 undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI2 is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI2 crystals in perovskite films is very important to improve device operational stability for diverse opto-electronic applications in the future.